Showing papers in "Journal of the Chemical Society, Faraday Transactions in 1982"
TL;DR: In this article, a series of Si, Al ordering schemes were constructed for phase-pure synthetic faujasites with Si/Al ratio in the range 1.19-2.75.
Abstract: High-resolution solid-state 29Si nuclear magnetic resonance (n.m.r.) spectra were recorded at 79.80 MHz, with rapid spinning of the sample at the magic angle to the external magnetic field, for a series of phase-pure synthetic faujasites with Si/Al ratio in the range 1.19–2.75. On the basis of previous work which related 29Si chemical shift to environment, signal intensities corresponding to Si(nAl) structural units (n= 0, 1, 2, 3, 4) could be quantitatively determined by accurate deconvolution of the spectrally well-resolved Gaussian peaks. Even though n.m.r. spectra do not by themselves provide direct evidence for Si, Al ordering beyond the first tetrahedral coordination shell, from the observed ratio of intensities of Si(nAl) units and from arguments based on crystal symmetry and electrostatic energy, a series of Si, Al ordering schemes was constructed. These schemes offer greater insight into the structure of the aluminosilicate framework than can be achieved at present by other techniques. Inter alia, they reveal that the Loewenstein rule, which forbids Al atoms to occupy neighbouring framework tetrahedral sites, is strictly obeyed for zeolites of the faujasite structure. Our results and conclusions can also explain observed discontinuites in the plot of the (cubic) unit-cell parameter against aluminium content in terms of Si, Al ordering within the framework. Moreover, the magic-angle-spinning nuclear magnetic resonance (m.a.s.n.m.r.) spectra provide an independent method of determining Si/Al ratios: ratios determined by X-ray fluorescence or analytical electron microscopy agree within a few per cent with those derived from m.a.s.n.m.r., thus lending further credence to the value of solid-state n.m.r. in studies of aluminosilicates. The effect of second-nearest (tetrahedral) neighbours on chemical shifts, as well as the spectral and structural characteristics of faujasites possessing “non-ideal” Si/Al ratios, are also discussed.
230 citations
TL;DR: In this paper, an electron acceptor (Fe3+) which is part of a reversible redox couple was described for the photo-oxidation of water with visible light, and the formal quantum efficiency for oxygen production at 405 nm was (3.1 ± 0.1)× 10−3 mol ein−1.
Abstract: WO3 powder photosensitized the oxidation of water by Fe3+ ions with blue light (400–450 nm). The formal quantum efficiency for oxygen production at 405 nm was (3.1 ± 0.1)× 10–3 mol ein–1. Oxygen production was partially inhibited by Fe2+, oxygen and high concentrations of Fe3+(> 10–2 mol dm–3). Depositing RuO2 on to the semiconductor powder produced a moderate increase in the rate of oxygen production, while deposits of Pt, Rh and Ru lead to a decrease in the rate of oxygen formation. Unlike most of the artificial systems which have been described for the photo-oxidation of water with visible light, this system operates with an electron acceptor (Fe3+) which is part of a reversible redox couple (Fe3+/Fe2+).
173 citations
TL;DR: In this paper, the authors investigated the removal of Pb2+ ions by four synthetic hydroxyapatites (S-1, S-2 and S-3) using both batch and column methods and found that the removal is due not only to an adsorption effect but also to an ion exchange reaction between the cations in solution and the Ca2+ ion of the apatites.
Abstract: The removal of [graphic omitted] h as Pb2+, Mn2+, Co2+ and Cu2+ in aqueous solution by four synthetic hydroxyapatites (S-1, S-2, S-3, S-4) has been investigated using both batch and column methods. The removal is due not only to an adsorption effect but also to an ion-exchange reaction between the cations in solution and the Ca2+ ions of the apatites. The order of the ions according to the amount exchanged was as follows: Pb2+ > Cu2+ > Mn2+≃ Co2+. Pb2+ ions were readily removed by the apatites and the maximum value for the exchange of Pb2+ ions was 230 mg per g of S-4 apatite. The apatites, particularly S-4, would seem to be possible agents for the removal of toxic Pb2+ ions. The selectivity of the apatites for the cations can be explained by considering the radii and the electronegativities of the ions.
170 citations
TL;DR: The wettability of flat quartz-crystal surfaces has been assessed by measurement at 25 °C of contact angle at the water-vapour/water-drop/quartz-plate three-phase line.
Abstract: The wettability of flat quartz-crystal surfaces has been assessed by measurement at 25 °C of contact angle at the water-vapour/water-drop/quartz-plate three-phase line. Plates pretreated by heating in vacuum gave angles, measured through the drop 1 min after removal from vacuum, of 0–44 ° as pretreatment temperature was increased from 200 to 1000 °C. Fully hydroxylated, therefore hydrophilic, quartz surfaces are progressively rendered hydrophobic by mutual condensation of surface hydroxyls to form siloxane bridges. Hysteresis was at a maximum after heating at 700–800 °C, indicating that maximum surface chemical heterogeneity was produced by heating in this temperature range. Plates methylated subsequent to heat treatment gave angles that were constant at ca. 80° up to treatment at 600 °C and that decreased from 80 to ca. 47 ° as the pretreatment temperature was further increased to 1000 °C. This variation with temperature is consistent with a mechanism for methylation in which only non-hydrogen-bonded surface hydroxyl groups on quartz are reactive towards the methylating reagent. Contact angles on both heat-treated and methylated plates were observed to decrease following extended exposure to water vapour.
159 citations
TL;DR: The thermal desorption spectra of ammonia, amines and pyridines chemisorbed on Cr2O3 as well as the infrared absorption spectra have been measured in order to elucidate the acidic properties of the surface.
Abstract: The thermal desorption spectra of ammonia, amines and pyridines chemisorbed on Cr2O3 as well as the infrared absorption spectra have been measured in order to elucidate the acidic properties of the surface. The adsorption of ammonia on the bare Cr2O3 surface at room temperature leads to the formation of two kinds of coordination species giving rise to desorption peaks at 443 and 543 K, and the bonding of the latter species to the surface is strengthened to cause a shift in the peak to 593 K when the adsorption temperature is raised to 473 K. Dimethylamine bonded to Lewis-acid sites on Cr2O3 causes the N—H bond-cleavage reaction when the sample is heated at 473 K. Pyridine is strongly held on the surface: half the pyridine molecules absorbed on Cr2O3 at room temperature remain on the surface even after evacuation at 773 K. The infrared study of these molecules suggests the formation of an α-pyridone-like complex.
153 citations
TL;DR: In this paper, the photochemical cleavage of water using u.v. light (λ < 400 nm) under conditions of room temperature and pressure was investigated using semiconductor powders (TiO2 and SrTiO3).
Abstract: n-Type semiconductor powders (TiO2 and SrTiO3) were used to sensitise the photochemical cleavage of water using u.v. light (λ < 400 nm) under conditions of room temperature and pressure. Although several methods of platinising these powders were used, one method in particular (involving precipitation of a platinum sol, by addition of an inert electrolyte, in the presence of the semiconductor powder) was found to produce an efficient photocatalyst for water reduction. Many different photocatalysts were tested for water reduction activity, using EDTA as an electron donor, and for water oxidation activity, using Fe3+ as an electron acceptor. In the absence of EDTA and Fe3+, u.v. irradiation of these photocatalysts liberated H2 but O2 evolution was not observed. Reasons for these observations are discussed.
147 citations
TL;DR: In this paper, the phase separation of aqueous solutions of some non-ionic surfactants, in particular poly(ethylene glycol) alkyl ethers like tetra-and hexa-decyl ether, is analyzed theoretically.
Abstract: The phase separation (clouding) of aqueous solutions of some non-ionic surfactants, in particular poly(ethylene glycol) alkyl ethers like tetra- and hexa-(ethylene glycol) decyl ether, is analysed theoretically. It is found that the location of the solubility gap is incompatible with a spherical form of the micelles. The phase boundary can be reproduced theoretically if the micelles are assumed to be rod shaped and flexible at high temperatures.According to the picture presented here, the main contribution to the intermicellar interaction comes from the contact between poly(ethylene glycol)(PEG) chains belonging to different micelles. As in the case of the pure PEG–water system, there are zones with increased water structuring around the PEG chains. The existence of a lower consolute phase boundary is an effect of the “solvation” force (a modified hydrophobic interaction) that originates from the overlap of such zones. Some further consequences of this kind of force in the micellar and the liquid-crystalline phases are briefly discussed. It can lead to an increase in aggregate size with temperature.
142 citations
TL;DR: The formation constant and absorption spectrum of the dimer of Rhodamine 6G have been evaluated: the trimer is not formed as discussed by the authors, and the geometric structure of the aggregate is determined using exciton theory.
Abstract: The formation constant and absorption spectrum of the dimer of Rhodamine 6G have been evaluated: the trimer is not formed. The geometric structure of the aggregate is determined using exciton theory. The thermodynamics of the dimerization process permit the study of association forces, justifying the structural differences from those exhibited by Rhodamine B dimers.
139 citations
TL;DR: In this article, a simple model based on the theory of interaction between electrical double layers was proposed to compare the shear-modulus data for polystyrene latices at various volume fractions and sodium chloride concentrations.
Abstract: Shear-modulus data for polystyrene latices at various volume fractions and sodium chloride concentrations are compared with a simple model based on the theory of interaction between electrical double layers. Stress relaxation and low shear-rate viscosity are also discussed in the context of particle interaction forces.
136 citations
TL;DR: In this article, an experimental investigation of the viscoelastic behavior of concentrated polystyrene latices is described. But the authors used creep compliance and shear-wave propagation to determine low stress viscosities and the latter to determine the shear modulus.
Abstract: Concentrated polystyrene latices form strongly interacting systems, and under appropriate conditions of particle radii, volume fraction and ionic strength the repulsive forces can be strong enough to give rise to measurable viscoelastic behaviour. In this paper an experimental investigation of the viscoelastic behaviour of concentrated latices is described. The methods used included creep compliance and shear-wave propagation. The former was used to determine the low stress viscosities and the latter to determine the shear modulus. The particle radii were varied from 26 to 98 nm and the volume fraction range was 0.2–0.4.
126 citations
TL;DR: The role of alcohol in the studied system is discussed and the rate of growth explained in terms of the theory of Burton, Cabrera and Frank in this paper, and it is shown that α-Fe2O3 formed by phase transformation from β-FeOOH precipitated first.
Abstract: Cubic particles of colloidal haematite of narrow size distributions were prepared by aging ferric chloride solutions in water + ethanol mixtures. It was shown that α-Fe2O3 formed by phase transformation from β-FeOOH precipitated first. The kinetics of this conversion were followed at different temperatures, pH and chloride concentrations. The role of alcohol in the studied system is discussed and the rate of growth explained in terms of the theory of Burton, Cabrera and Frank.
TL;DR: In this article, the relationship of cell potential to degree of oxidation, coulombic and energy efficiencies, constant-current discharge characteristics, energy density and maximum power density of a partly oxidized polyacetylene, [CH(ClO4)y](y [graphic omitted] 0.07), cathode in a cell of the type [CH (ClO 4)y]x|Li ClO4|Li are discussed.
Abstract: The relationship of cell potential to degree of oxidation, coulombic and energy efficiencies, constant-current discharge characteristics, energy density and maximum power density of a partly oxidized polyacetylene, [CH(ClO4)y](y [graphic omitted] 0.07), cathode in a cell of the type [CH(ClO4)y]x|LiClO4|Li are discussed. Coulombic efficiencies ranging from 100 to 86% and energy efficiencies ranging from 81 to 68% during a charge–discharge cycle are found at oxidation levels ranging from 1.54 to 6.0%. Energy densities of ca. 255 W h kg–1(based on the weights of the electroactive materials involved in the discharge process) are obtained for 7.0% oxidized polyacetylene cathodes under constant-current discharge conditions. Maximum power densities of ca. 30 kW kg–1 are observed.
TL;DR: In this paper, the reaction of aluminium(III) sulphonated phthalocyanine (AlSP) with benzohydroquinone (BQH2), methyl viologen (MV) and anthraquinone-2,6-disodium sulphonate (AQDS) was studied.
Abstract: Conventional and nanosecond flash photolysis has been used to study the excited-state reactions of aluminium(III) sulphonated phthalocyanine (AlSP) with benzohydroquinone (BQH2), methyl viologen (MV), benzyl viologen (BV) and anthraquinone-2,6-disodium sulphonate (AQDS). The fluorescent state of AlSP was quenched by all of these reagents at rates close to the diffusion-controlled limit; however, no separated electron-transfer products were detected from the reaction with BV, MV or AQDS despite the electrostatic repulsion between the phthalocyanine and semiquinone radical anions. In contrast, quenching by BQH2 produced a transient (τ≃ 100 ns), which is attributed to a long-lived geminate ion pair, from which separated ions escaped.The triplet state of AlSP did not react with AQDS, BV or MV, but it was quenched by oxygen and BQH2. Separated ions were produced three times as efficiently from the triplet reaction of AlSP with BQH2 than from the excited-singlet-state reaction.
TL;DR: In this article, a mechanism was proposed for the interaction of oxygen with preadsorbed hydrogen envisaging an electron-transfer process from H− ions to oxygen molecules to give O−2 paramagnetic species.
Abstract: Hydrogen adsorption on high-surface-area alkaline-earth oxides has been studied by i.r. spectroscopy at 300 and 78 K. In the former case heterolytic dissociation of H2 molecules produces hydride and hydroxy groups which are partially reversible upon evacuation. Low-temperature hydrogen adsorption appears to be undissociative. The active sites are cation–anion couples in low coordination on the surface.On the basis of i.r. evidence a mechanism is proposed for the interaction of oxygen with preadsorbed hydrogen envisaging an electron-transfer process from H– ions to oxygen molecules to give O–2 paramagnetic species.
TL;DR: In this paper, a self-consistent set of Arrhenius parameters for the 1.5p H-atom transfer in alkylperoxy radicals was derived from values of rate constants at 480 °C.
Abstract: The reactions of neopentyl radicals in an oxidising environment have been studied by adding neopentane to slowly reacting mixtures of H2+ O2 over the temperature range 380–520 °C. Over a wide range of mixture composition, the only detectable initial products at these temperatures are 3,3-dimethyloxetan (DMO), acetone, i-butene, methane, and formaldehyde. A relatively simple mechanism involving the formation of neopentylhydroperoxide (QOOH) radicals gives a quantitative interpretation of the product yields. Although the major source of i-butene is the C—C homolysis of neopentyl radicals, a significant proportion is formed in reaction (6)(CH3)2C(CH2OOH)CH2→(CH3)2CCH2+ HCHO + OH. (6)From measurements of the product ratios ([acetone]+[DMO])/[i-butene] and [acetone]/[DMO] at each temperature used, Arrhenius parameters have been determined for a number of the elementary steps. The recommended value of k3= 1.20 × 1013 exp (– 120 kJ mol–1/RT) S–1 for the 1,5p H-atom transfer in neopentylperoxy radicals is compared with previously determined parameters for the 1,4p H-atom transfer in ethylperoxy radicals. With the Arrhenius expressions for these two transfers as a basis, thermochemical calculations are used to obtain a self-consistent set of Arrhenius parameters from values of rate constants at 480 °C for primary, secondary and tertiary H-atom transfers in alkylperoxy radicals involving ring sizes in the transition state varying from 4 to 8 (CH3)3CCH2O2→(CH3)2C(CH2OOH)CH2. (3)
TL;DR: In this paper, the diffusion of rod-like molecules is studied in the case of high density and it is argued that when one goes to densities higher than those permitting rotational freedom for the rods, the diffusion will depend critically upon the cooperation of the others.
Abstract: The diffusion of rod-like molecules is studied in the case of high density. It is argued that when one goes to densities higher than those permitting rotational freedom for the rods, the diffusion of one rod will depend critically upon the cooperation of the others. A calculation of this effect using mean-field theory yields a transition to immobility at a certain density, and this is offered as a simple model akin to a glass transition.
TL;DR: In this paper, the authors measured self-diffusion coefficients for liquid n-hexane measured by the n.m. spin-echo technique at 223, 248, 273, 298 and 333 K at pressures up to 400 MPa.
Abstract: Self-diffusion coefficients for liquid n-hexane measured by the n.m.r. spin–echo technique are reported at 223, 248, 273, 298 and 333 K at pressures up to 400 MPa. Owing to a marked non-linear volume dependence, the results could not be directly fitted to the rough hard-sphere model of diffusion, but reduced diffusion coefficients were fitted as a function of a reduced volume, expressed as the ratio of the molar volume to a ‘hard-core volume’. The temperature dependence of an equivalent hard-sphere diameter calculated from this hard-core volume was found to be adequately represented by an equation originally proposed by Protopapas, Andersen and Parlee for liquid metals: σ=σ0[1–B(T/Tm)½]. The constant B, 0.072, is of similar magnitude to that found to fit CH4 and C2H4 self-diffusion data, 0.069, and is consistent with diameters derived from the viscosity measurements of Brazier and Freeman. Between 273 and 333 K where the diffusion and viscosity results overlap, the group Dηt is constant, at a given temperature, with t= 0.97: consequently the Stokes–Einstein equation is not obeyed.
TL;DR: In this article, the reaction of allyl radicals with oxygen has been studied using argon fluoride laser flash photolysis coupled with ultraviolet absorption detection, and the temperature dependences of both the forward and reverse rate constants, k1 and k-1, were obtained.
Abstract: The reaction of allyl radicals with oxygen has been studied using argon fluoride laser flash photolysis coupled with ultraviolet absorption detection. At temperatures between 382 and 453 K it was possible to observe directly the establishment of equilibrium between allyl, oxygen and allyl peroxy (C3H5+ O2⇌ C3H5O2). From analysis of the allyl radical decay signals the temperature dependences of both the forward and reverse rate constants, k1 and k–1, were obtained. The temperature dependence of the equilibrium constant, Kp=k1/(k–1RT) gave a value for ΔS⊖298 of –(122 ± 5) J mol–1 K–1 and for ΔH⊖298 of –(76.2 ± 2.1) kJ mol–1. The equilibrium constants were shown to be pressure-independent, although both k1 and k–1 were below their high-pressure limits. Above 453 K the form of the decay and the apparent value of the equilibrium constant deviate from the lower-temperature behaviour because of the onset of an additional reaction channel.
TL;DR: In this article, the effect of CO2 addition on a ZnO Kadox-15 powder has been studied by Fourier transform infrared spectroscopy, and it was deduced that the splitting is due to a coupling between two linear species held by the same Zn2+ ion.
Abstract: Adsorption of carbon dioxide on a ZnO Kadox-15 powder has been studied by Fourier transform infrared spectroscopy. The following surface species are formed: bidentate carbonates, polydentate carbonates which appear with time or heating, hydrogenocarbonates and linear CO2 species. Moreover, a band at 1546 cm–1 could correspond to carboxylates reversibly adsorbed at room temperature. Attention is paid to the effect of CO2 addition which splits the νa(OCO) and δ(CO2) bands due to linear species and shifts the bidentate carbonates band from 1595 to 1615 cm–1. Taking account of the band va(O13CO)(in natural abundance), we deduce that the splitting is due to a coupling between two linear species held by the same Zn2+ ion. We propose that such Zn2+ ions that are two-fold coordinate are situated on the edges formed by the (0001)(10text-decoration:overline10) planes.
TL;DR: When sodium is added at mole fractions of ca. 10−4 to fuel-rich premixed hydrogen/oxygen/nitrogen flames of temperature T between 1800 and 2200 K, accelerated recombination of excess hydrogen atoms and hydroxy radicals is observed.
Abstract: When sodium is added at mole fractions of ca. 10–4 to fuel-rich premixed hydrogen/oxygen/nitrogen flames of temperature T between 1800 and 2200 K, accelerated recombination of excess hydrogen atoms and hydroxy radicals is observed. The effect is quantitatively interpreted in terms of the reactions NaOH + H [graphic omitted] Na + H2O, (1), Na + OH + M [graphic omitted] NaOH + M. (2) Rate coefficients are as follows: log10(k1/cm3 molecule–1 s–1)=– 10.75 ± 0.5 – 430 T–1, log10(k–1/cm3 molecule–1 s–1)=– 9.39 ± 0.5 – 9500 T–1, log10(k2/cm6 molecule–2 s–1)=– 26.30 ± 0.4 – log10T, log10(k–2/cm3 molecule–1 s–1)=– 2.60 ± 0.4 – log10T– 17 100 T–1.
TL;DR: In this article, the authors treated the kinetics of the extraction of a soluble constituent from tea leaf by a steady-state model and found that the last step was not the rate-determining one.
Abstract: The kinetics of the extraction of a soluble constituent from tea leaf have been treated by a steady-state model. This leads to an overall rate constant made up of 3 main contributions which arise from the diffusion of the constituent through the leaf, its transfer across the leaf/water interface, and its diffusion away through the Nernst layer. That the last step was not the rate-determining one was shown by rotating-disc experiments. Koomsong tea dust was glued on to large discs and the rate of caffeine extraction at 80 °C measured at various rotation speeds. The rate was found to be independent of the speed. The viability of the experimental procedure was checked using discs coated with similarly-sized copper powder: here the rate of attack by dilute dichromate increased with increasing rotation speed in semi-quantitative agreement with the Levich equation.
TL;DR: In this article, the mechanism of the hydrogenation of carbon monoxide to hydrocarbon products over ruthenium catalysts has been investigated, and it is concluded from the behavior of the surface species under the reaction conditions that all the hydrocar products are produced via dissociatively adsorbed CO with no CO insertion, leading to the conclusion that it comprises the conversion of C1 intermediates to the reaction products.
Abstract: The mechanism of the hydrogenation of carbon monoxide to hydrocarbon products over ruthenium catalysts has been investigated. By measuring the adsorption and observing the infrared adsorption spectra of the adsorbed species during the course of the reaction, the accumulation of surface hydrocarbon species on the ruthenium catalysts was confirmed, whereas most of the surface was covered by molecularly adsorbed carbon monoxide. The reaction intermediate was examined using carbon-13. The reactivity of deposited carbon formed by the Boudouard reaction has also been studied using carbon-13. It is concluded from the behaviour of the surface species under the reaction conditions that all the hydrocarbon products are produced via dissociatively adsorbed CO with no CO insertion. The rate-determining step has been examined, leading to the conclusion that it comprises the conversion of C1 intermediates to the reaction products.
TL;DR: A nuclear magnetic resonance study of xenon adsorbed on Pt-NaY zeolites has made it possible, first, to prove that chemisorption of hydrogen occurs homogeneously on all particles of similar size, and, secondly, to determine the number of platinum particules and therefore their mean size as mentioned in this paper.
Abstract: A nuclear magnetic resonance study of xenon adsorbed on Pt-NaY zeolites, with or without pre-chemisorbed hydrogen, has made it possible, first, to prove that chemisorption of hydrogen occurs homogeneously on all particles of similar size, and, secondly, to determine the number of platinum particules and therefore their mean size. This technique seems highly useful when particles contain very few atoms and cannot be detected by electron microscopy.
TL;DR: In this paper, a first-order reaction with respect to undissociated hydrogen peroxide was shown to be possible in 1.5 mol dm-dm-3 KOH and 1.2 mol Dm-m −3 NaOH solutions.
Abstract: Hydrogen peroxide decomposition in 1–5 mol dm–3 KOH and 1–2 mol dm–3 NaOH solutions is a first-order reaction with respect to undissociated hydrogen peroxide. The decomposition is catalysed by compounds of heavy metals (Fe, Cu) present as trace impurities in these solutions and is first order with respect to them. The hydroxyl ion concentration exerts a significant effect on the decomposition rate, which has been explained by its influence on the activity of catalysing species having colloidal character, which are probably the active sites for the decomposition.
TL;DR: In this paper, the nitrogen-15 spectra of ammonia, trimethylamine, pyridine and acetonitrile molecules were measured at 9.12 MHz by means of the conventional Fourier transform n.m.r. technique.
Abstract: Nitrogen-15 n.m.r. spectroscopy has been applied to the study of the interactions between solid surfaces and molecules adsorbed on them. Nitrogen-15 spectra of ammonia, trimethylamine, pyridine and acetonitrile molecules sorbed in various zeolites were measured at 9.12 MHz by means of the conventional Fourier-transform n.m.r. technique. In all measurements carried out, substances were employed which were enriched with nitrogen-15 nuclei (ca. 95 %).The resonance shifts depend strongly on the nature of adsorption sites which may occur in the zeolites (e.g. Na+ cations, Bronsted- and Lewis-acid sites).The results clearly reveal the advantage of nitrogen-15 n.m.r. investigations in characterizing acidic properties in comparison with the carbon-13 n.m.r. measurements performed until now on adsorbate–adsorbent systems and emphasize that nitrogen-15 spectroscopy may become a powerful tool for the study of surface phenomena.
TL;DR: In this article, self-diffusion measurements for acetonitrile in the temperature range 238-343 K at pressures up to 300 MPa were reported, and a correlation approach based on Enskog smooth hard-sphere theory was found to be less sensitive to the dipole-dipole interactions.
Abstract: Self-diffusion measurements are reported for acetonitrile in the temperature range 238–343 K at pressures up to 300 MPa. Lack of reliable high-pressure density data has restricted tests of theory to the temperature range 268–328 K. Over that range the results show a breakdown of rough hard-sphere theory which is attributed to the strongly dipolar nature of the acetonitrile molecule. A correlation approach based on Enskog smooth hard-sphere theory is found to be less sensitive to the dipole–dipole interactions.
TL;DR: The reactive nature of the polycrystalline zinc oxide surface has been investigated using methyl formate and ethanol as probe molecules; these have shown the dominant influence of the cation-anion dual site as discussed by the authors.
Abstract: The reactive nature of the polycrystalline zinc oxide surface has been investigated using methyl formate and ethanol as probe molecules; these have shown the dominant influence of the cation–anion dual site. Methyl formate decomposes during adsorption at the dual site to adsorbed methoxy and formate species. On temperature programming these show identical kinetics and reaction pathways to the methoxy and formate species observed previously after the adsorption of formaldehyde or methanol. Adsorption of ethanol at the dual site results in the formation of an adsorbed ethoxy species. On temperature programming, at 510 K, the zinc oxide surface abstracts one of the β-hydrogen atoms to form an unstable C2H4O fragment which decomposes mainly (90%) to ethylene; a small amount (ca. 10%) of the desorbing material which is either acetaldehyde or ethylene oxide (these cannot be distinguished by mass spectrometry) is desorbed coincidently at this temperature in an isomerization/desorption step. Despite the ethylene formation being a dehydration reaction, no water is observed in the desorption spectrum, the ethoxy oxygen species being left on the surface during decomposition.
TL;DR: In this article, the enthalpic effect of hydrophobic hydration of the solutes was calculated by application of a clathrate-like hydration model, and it was concluded that methyl substitution at the N atom gives changes in the enthpies of transfer from H2O to DMF which are different from those caused by methyl substitutions at the C atom.
Abstract: Enthalpies of solution of five alkyl-substituted ureas and seven different amides have been determined at 298.15 K in mixtures of NN-dimethylformamide (DMF) and H2O. Methyl substitution of the ureas causes changes in the enthalpies of transfer from H2O to DMF which show that either side of these molecules is solvated independently. From the measurements on the amides it is concluded that methyl substitution at the N atom gives changes in the enthalpies of transfer from H2O to DMF which are different from those caused by methyl substitution at the C atom. Analysis of the data in the mixed solvent shows that introduction of more or longer alkyl groups into the molecules makes both ureas and amides considerably more hydrophobic. After accounting for the influence of the NH protons, in both ureas and amides, the enthalpic effect of hydrophobic hydration of the solutes was calculated by application of a clathrate-like hydration model. Enthalpic effect of N-substituted methyl groups in ureas and amides prove to be virtually equal. The variation in the enthalpic effects of hydrophobic hydration with the number of C atoms in the n-alkyl group is comparable to that found for alcohols and amines.
TL;DR: In this article, a detailed study of the reductive dissolution of NiFe2O4 by V(pic)3 is reported, showing that the reaction follows a cubic rate law and exhibits a first-order dependence on [VII, indicating that the rate-determining step involves attack on FeIII ions at the oxide surface.
Abstract: A detailed study of the reductive dissolution of NiFe2O4 by V(pic)–3 is reported. The kinetics of the reaction follow a cubic rate law and exhibit a first-order dependence on [VII], indicating that the rate-determining step involves attack by V(pic)–3 on FeIII ions at the oxide surface. Dependences on [H+] and [free picolinate] are attributed to adsorption of these species at surface sites according to a simple model based on the Langmuir adsorption isotherm. Buffer and surfactant concentration had little effect. Some pitting etc. of the oxide surface occurs as the dissolution proceeds, although there appears also to be some general surface attack. The nature of the surface sites and subsequent steps in the dissolution process are discussed.
TL;DR: The thermotropic liquid crystal mesophase formed by di-isobutylsilanediol has been known for over twenty-five years, but its nature and structure have remained uncertain and the physical characteristics of the phase appeared unique and quite different from those of any other recognised type of liquid crystal as discussed by the authors.
Abstract: The thermotropic liquid crystal mesophase formed by di-isobutylsilanediol has been known for over twenty-five years, but its nature and structure have remained uncertain and the physical characteristics of the phase appeared unique and quite different from those of any other recognised type of liquid crystal. In a previous preliminary publication, it was suggested that the mesophase of di-isobutylsilanediol is in fact of the discotic type only very recently characterised in certain organic systems. Here we describe more fully our grounds for this suggestion. A model is proposed for the structure of this mesophase; this offers an explanation as to why other dialkylsilanediols do not behave similarly. A reinterpretation of earlier X-ray crystallographic data led us to suggest that the mesophase consists of hydrogen-bonded dimers. The model is supported by data from optical microscopy, thermal analysis and miscibility studies. The optical textures shown by the mesophase and those observed at the crystal mesophase boundary are discussed and compared with those of the discotic phases of the benzene hexa-n-alkanoates.